1. Field of Invention
The present invention relates to a dual light source voltage modulated reciprocal control circuit. More particularly, the present invention relates to a dual light source voltage-modulated reciprocal control circuit for a scanner.
2. Description of Related Art
A scanner requires a light source to conduct a document scanning. In general, a scanner has two sets of lights for document scanning, namely, a set of back lights for scanning of ordinary opaque documents and a set of cover lights for scanning transparent documents.
FIG. 1 is a block diagram showing a dual light source circuit in a conventional scanner. As shown in FIG. 1, square wave signals SV1 and SV2 are transmitted from an application specific integrated circuit 102 to a first voltage-modulated circuit 104 and a second voltage-modulated circuit 106 respectively. Pulse width of the square wave signals SV1 and SV2 can be modulated (for example, within the range 15%˜80%). The larger the pulse width of the square wave signals SV1 and SV2, the greater will be the magnitude of the modulated voltages MV1 and MV2 each having a direct current (dc) square wave profile submitted from the first voltage-modulated circuit 104 and the second voltage-modulated circuit 106.
The first voltage-modulated circuit 104 and the second voltage-modulated circuit 106 submit the modulated voltages MV1 and MV2 to a back light driving circuit 108 and a cover light driving circuit 110 respectively. The back light driving circuit 108 and the cover light driving circuit 110 are dc-to-ac inverters capable of converting a direct current (dc) voltage into an alternating (ac) voltage. The back light driving circuit 108 issues an alternating voltage INV1 to a back light 112 and the cover light driving circuit 110 issue an alternating voltage source INV2 to a cover light 114. Hence, either the back light 112 or the cover light 114 is triggered to conduct a document scanning but not both.
However, most conventional scanners having a dual light source circuit employ two sets of voltage-modulating circuits. With the deployment of two voltage-modulating circuits, more area on a printed circuit board is required to house component devices. Ultimately, a greater hardware cost is incurred.